The long-term objective is to develop dependable ways of assessing radiation risks from tissue-incorporated radionuclides and to provide a more rational basis for the design of radiation therapy protocols. Specifically, we hope to demonstrate the feasibility of using perturbed angular correlation (PAC) spectroscopy to characterize radiation damage of biomolecules at the site of electron capture decay. An innovative new design for a PAC spectrometer will be implemented. The unique feature of the proposed instrument is the capability to generate two spectra (K x-ray coincidence and anticoincidence) for each sample. These spectra correspond to two distinct levels of highly localized energy deposition around the decay site. The feasibility of this new spectrometer depends largely upon the development of a fast risetime, high absolute efficiency K x-ray detector. This detector must interfere minimally with gamma-rays and function effectively in the context of the overall spectrometer design. On the basis of a careful study of the operating conditions of the temporary detector used at the University of Massachusetts to obtain preliminary data, the above three K x-ray detector characteristics are probably obtainable for both the 111In and 77Br nuclides. Various labeled molecules will be investigated with K x-ray coincidence PAC spectroscopy. Initially we will explore the phenomenology of radiation damage in PAC spectra. The features, if any, in the PAC spectra which depend upon locally absorbed dose will manifest themselves as differences in the K x-ray coincidence and anticoincidence spectra.